Nonablative state changeable data storage systems, for example, optical data storage systems, record information in a state changeable material that is switchable between at least two detectable states by the application of projected beam energy thereto, for example, optical energy. Data may be stored reversibly or irreversibly. When optical energy is used to determine the state of the state changeable material, the measured property may be the reflectivity. Reflectivity is a function of wavelength, material thickness, and index of refraction, where the index of refraction is a function of the state of the material.
State changeable data storage material is incorporated in a data storage device having a structure such that the data storage material is supported by a substrate and encapsulated in encapsulants. In the case of optical data storage devices the encapsulants include, for example, anti-ablation materials and layers, thermal insulation materials and layers, anti-reflection materials and layers, reflective layers, and chemical isolation layers. Moreover, various layers may perform more than one of these functions. For example, anti-reflection layers may also be anti-ablation layers and thermal insulating layers. The thicknesses of the layers, including the layer of state changeable data storage material, are optimized to minimize the energy necessary for state change and maximize the high contrast ratio, high carrier to noise ratio, and high stability of state changeable data storage materials.
The state changeable material is a material capable of being reversibly or irreversibly switched from one detectable state to another detectable state or states by the application of projected beam energy thereto. State changeable materials are such that the detectable states may differ in their morphology, surface topography, relative degree of order, relative degree of disorder, electrical properties, optical properties including indices of refraction and reflectivity, or combinations of one or more of these properties. The state of state changeable material is detectable by the electrical conductivity, electrical resistivity, optical transmissivity, optical absorption, optical refraction, optical reflectivity, or combinations thereof.
Formation of the data storage device includes deposition of the individual layers, for example by evaporative deposition, chemical vapor deposition, and/or plasma deposition. As used herein plasma deposition includes sputtering, glow discharge, and plasma assisted chemical vapor deposition.
Tellurium based materials have been utilized as state changeable materials for data storage where the state change is a structural change evidenced by a change in reflectivity. This effect is described, for example, in J. Feinleib, J. deNeufville, S.C. Moss, and S.R. Ovshinsky, "Rapid Reversible Light-Induced Crystallization of Amorphous Semiconductors," Appl. Phys. Lett., Vol. 18(6), pages 254-257 (Mar. 15, 1971), and in U.S. Pat. No. 3,530,441 to S.R. Ovshinsky for Method and Apparatus For Storing And Retrieving Of Information. A recent description of tellurium-germanium-tin systems, without oxygen, is in M. Chen, K.A. Rubin, V. Marrello, U.G. Gerber, and V.B. Jipson, "Reversibility And Stability of Tellurium Alloys for Optical Data Storage," Appl. Phys. Lett., Vol. 46(8), pages 734-736 (Apr. 15, 1985). A recent description of tellurium-germanium-tin systems with oxygen is in M. Takanaga, N. Yamada, S. Ohara, K. Nishiciuchi, M. Nagashima, T. Kashibara, S. Nakamura, and T. Yamashita, "New Optical Erasable Medium Using Tellurium Suboxide Thin Film," Proceedings, SPIE Conference on Optical Data Storage, Arlington, VA, 1983, pages 173-177.
Tellurium based state changeable materials, in general, are single or multi-phased systems (1) where the ordering phenomena include a nucleation and growth process (including both or either homogeneous and heterogeneous nucleations) to convert a system of disordered materials to a system of ordered and disordered materials, and (2) where the vitrification phenomenon includes melting and rapid quenching of the phase changeable material to transform a system of disordered and ordered materials to a system of largely disordered materials. The above phase changes and separations occur over relatively small distances, with intimate interlocking of the phases and gross structural discrimination, and are highly sensitive to local variations in stoichiometry.
A major limitation of optical data storage devices is lack of reproducability of the contrast from one production run to the next. The contrast is the difference between the reflectivity of the optical data storage material in one state and its reflectivity in another state. Reflectivity is affected by interference phenomena. The reflectivity of the optical data storage medium is dependent on the thickness of the film of the optical data storage medium, the wavelength of the projected beam optical energy used for interrogation, and the index of refraction of the optical data storage medium.
The interrogation means is typically a monochromatic light source. The wave length of this monochromatic light source defines the wave length for interference phenomena.
The index of refraction can be controlled by control of the chemical composition of the medium. This includes control of the composition of, for example, sputtering targets, evaporation sources, and chemical vapor deposition gases.
Thickness is a more difficult variable than composition to control. Thickness of deposited thin films have heretofore been controlled by, for example, simultaneously coating the substrate to be coated and an oscillator. The oscillator frequency is a indirect function of coating thickness. The difference in oscillator frequency from the beginning of a deposition run to the end of the deposition run can be correlated with the deposition thickness. The deposition thickness then determines the reflectivity of the deposition for a given refractive index.
Thickness may also be controlled by measuring the optical thickness or optical density of the deposit. For example, U.S. Pat. No. 3,773,548 to Baker, et al for METHOD OF MONITORING THE RATE OF DEPOSITING A COATING SOLELY BY ITS OPTICAL PROPERTIES describes a process where coating thickness is controlled by measuring the optical density of an evaporated coating on a continuously advancing substrate. In Baker the optical density of a point is measured only once. This measurement is used to control the deposition rate at a prior deposition station.
Thickness may also be measured by determing the interference fringes of the coating, as described, for example, by Alvin Goodman "Optical Interference Method for the Approximate Determination of Refractive Index and Thickness of a Transparent Layer", Applied Optics, Vol 17 (No.17), pages 2779-2787 (September 1978), and R. D. Pierce and W. B. Venard, "Thickness Measurements of Films On Transparent Substrates By Photoelectric Detection of Interference Fringes", Rev. Sc. Instrum., Vol. 45 (No. 1), pages 14-15 (January 1974).